Turn off screensaver if connected to a server (LXQt)

**Status**: done for macOS, Linux is missing

If you run Jamulus on Debian 10 under LXQt the screensaver automatically starts if Jamulus is connected to a server. Same problem on macOS (probably BigSur). It should be possible to inform the system that the application is active see the windows docs on this: https://docs.microsoft.com/de-de/windows/win32/api/winbase/nf-winbase-setthreadexecutionstate?redirectedfrom=MSDN

It probably depends on the OS how to manage this.

#260

Start by reviewing the existing macOS implementation and issue #260, then trace where Jamulus detects an active server connection. Investigate the OS-specific mechanism needed for Linux, especially Debian 10 with LXQt. Done means the screensaver stays inactive while connected to a server, without regressing the existing macOS behavior.

Written by the indexing model from the issue text.